When you test the system air-loss rate, the maximum acceptable leakage rates per minute after the initial drop are
- 7 kPa (1 psi) for single vehicles and 21 kPa (3 psi) for tractor-trailers.
- 34 kPa (5 psi) for single vehicles and 69 kPa (10 psi) for tractor-trailers.
- 34 kPa (5 psi) for single vehicles and 41 kPa (6 psi) for tractor-trailers.
- 21 kPa (3 psi) for single vehicles and 28 kPa (4 psi) for tractor-trailers. ✓
Answer: D. 21 kPa (3 psi) for single vehicles and 28 kPa (4 psi) for tractor-trailers.
The maximum acceptable leakage is 21 kPa (3 psi) per minute for a single vehicle and 28 kPa (4 psi) per minute for a tractor-trailer.
Why this is the answer
The MTO air brake chart sets a different allowable drop for each vehicle combination. A straight truck, tractor or bus may lose no more than 21 kPa (3 psi) in one minute; a tractor with a trailer may lose no more than 28 kPa (4 psi). The test is done with normal pressure, engine off, spring brakes released and the service brake held fully applied. The initial pressure drop when you first apply the brake is not part of the leak-rate result; what matters is whether the pressure keeps falling faster than the chart allows.
Why the other answers are wrong
- A. 7 kPa (1 psi) for single vehicles and 21 kPa (3 psi) for tractor-trailers. — Those figures are too low; the chart allows 21 kPa for a single vehicle, not 7 kPa, and 28 kPa for a tractor-trailer, not 21 kPa.
- B. 34 kPa (5 psi) for single vehicles and 69 kPa (10 psi) for tractor-trailers. — Those values are much higher than the chart permits for either a single vehicle or a tractor-trailer.
- C. 34 kPa (5 psi) for single vehicles and 41 kPa (6 psi) for tractor-trailers. — The single-vehicle figure is too high, and the tractor-trailer figure does not match the chart's 28 kPa limit.
Remember it
Match the limit to what is hitched: the more trailers, the higher the allowed drop (3, 4, 6 psi).